In systems and processes in which oxygen-rich gas is employed in the oxidation of biological or other organic materials, as in fermentation processes, consumption of oxygen is quite small and the reactor effluent contains a large amount of unreacted O.sub.2. In order to make such processes economically efficient, one needs to recover and recycle the O.sub.2 in the gaseous effluent of the reaction. For example, in the fermentation of a biomass one may employ an oxygen-rich treating gas containing for example about 50% O.sub.2 and about 50% N.sub.2. The effluent from the fermentation may contain about 5% CO.sub.2, 50% N.sub.2 and 45% O.sub.2. To recycle this effluent without contaminant build-up, it is necessary that the CO.sub.2 be first removed.